Why digital isolators use isolated DC-DC converters

The iCoupler® digital isolator with integrated isolated power isoPower® features an isolated dc-to-dc converter that can switch relatively large currents from 125 MHz to 200 MHz. Working at these high frequencies may increase concerns about electromagnetic radiation and conducted noise.

Although, our application note on the official website "IonPower Device Radiation Control Recommendations" provides guidelines for circuits and layouts that minimize radiation. Practice has been shown to reduce peak emissions by more than 25 dB through circuit optimization (reducing load current and supply voltage) and using splicing capacitors across the isolation barrier (through PCB internal capacitors).

But what if the design has multiple isoPower devices and the layout is very dense? Is it still possible to significantly reduce radiation? Here, we will provide some general guidelines for such situations.

The inner layer stitching capacitor is the most advantageous because it can build a low inductance structure. In the case where the overall PCB area is limited, it is a good way to use a multi-layer PCB. It is practical to use as many layers as possible while overlapping the power and ground planes (reference layers) as much as possible. Figure 1 shows an example of a stack.

The buried layer (3, 4 layers on the primary side and 2 to 5 layers on the secondary side) can carry power and ground current. The overlap across the isolation barrier (eg, layer 4 GND on the primary side and layer 3 V Iso on the secondary side) can form an ideal splicing capacitance. Multiple overlaps can be formed by stacking multiple layers of PCBs to increase overall capacitance. In order to maximize the capacitance, the thickness of the PCB dielectric material between the reference layers must also be reduced.

Another layout technique is to overlap the layers of adjacent isoPower channels. Figure 2 shows an example with four adjacent channels.

In this example, each output domain is isolated from other domains, but we can still take advantage of some overlap capacitors. Figure 3 shows this stack, and each isoPower device can be seen to increase capacitance and the connection of adjacent isolation regions.

It must be ensured that the internal and external clearance requirements are consistent with the final application. Ferrite beads can also be used to provide filtering on any cable connection, reducing the antenna effect that can cause radiation.

to sum up

How to minimize the radiation of multiple isoPower devices in a dense PCB layout? Please refer to the following points:

Minimize power requirements per channel

Build stitching on multiple PCB layers

It is practical to use as many PCB layers as possible

Use the thinnest dielectric between each reference layer

Connect between adjacent domains

Ensure that internal and external creepage distances still meet the requirements

Filter on cable connection